OK, so here goes...
Reason 1 - I had rather a lot of sand on there. Probably 10kg or so. It's worth getting rid of that amount of weight, probably.
Reason 2 - People have told me that by leaving mud, sand etc... stuck to the underside (and side) of my vehicle, it'll make it corrode faster.
However, my logical brain tells me that although sand and mud will keep water in contact with the metal for longer without letting it dry out, it will also stop the air getting to it. Given that water + air = rust, which is worse? Allowing the oxygen in, or not letting the water escape?